MEMS devices integrate very small mechanical devices with semiconductors to form sensors (temperature, pressure, gas, moisture, and motion), accelerometers, valves, gears, actuators, and micromirror devices. MEMS are often sensitive to the environment and may require a hermetically sealed package to be isolated.
A package comprises a MEMS device mounted on a packaging substrate such as plastic or ceramic, wires attached to bond pads, and a cover to enclose the device. U.S. Pat. No. 7,491,567 B2 describes one method for enclosing the MEMS by inserting the supporting substrate with mounted device into a cavity formed by the packaging. Anther method of packaging places a cover over the supporting substrate and mounted device and attaches the cover to the substrate.
A digital micromirror device (DMD), such as a Texas Instruments DLP® micromirror device, is a type of MEMS device which uses an array of individually positionable mirrors to project an image onto a display panel. The array of mirrors is fabricated above CMOS substrate wafers using a material such as silicon. Each device typically comprises multiple mirrors and multiple devices are formed on the wafer substrate in a grid array pattern. Scribe lines are etched onto the wafer substrate and form clearly defined lines which can be used to more easily singulate each packaged device. The bottom surface of the CMOS supporting substrate is bonded to another substrate, typically silicon, for additional mechanical strength. Upon completion of the MEMS fabrication process, wires are attached from the device to the bond pads and transmit signals. The bonding wires are easily damaged or dislodged. Various methods can be used to contain semiconductor and micromirror devices and bonding wires, which are also attached to substrates. One method is to deposit encapsulation material above the device and wires to enclose and cover the assembly. Various types of covers can be placed around the device and wires. The substrate is singulated once the devices are packaged.
In the case of a DMD, the package protects the micromirror device while also forming an optically transparent window above the device. The device is able to receive an incoming signal and project an image through the optically transparent window and onto a display panel. The cover surrounding the micromirror device protects the bonding wires and the DMD but also prevents extraneous illumination from being incident onto the mirrors arrayed on the device.
FIG. 1 (Prior Art) is an illustration of an “ON” mirror 102 and an “OFF” mirror 104.
The mirror 102 is in an “ON” state. It is tilted towards a pad 106 on the silicon substrate 108. Light 110 is incident onto the mirror 102 and reflected light 112 is projected onto a display system to form an image. The “ON” state is equivalent to a digital “1”.
The mirror 104 is in an “OFF” state. The mirror 104 is tilted toward a pad 114 on the substrate 108. Light 116 is incident upon the mirror 104 and reflected light 118 is projected away from the display system. The “OFF” state is equivalent to a digital “0” because no image is sent for display.
FIGS. 2A-B are drawings of a packaged micromirror device 200.
FIG. 2A (Prior Art) shows a top cover 202 attached to a package 204 comprised of a material such as plastic, metallic, or ceramic. The cover 202 is typically rectangular. The package 204 supports the cover 202 above the micromirror device 208. The cover 202 may be comprised of a material such as plastic, metallic, or ceramic and is attached to the supporting package 204 prior to singulation. An optically transparent window 206 is embedded within an opening in the top surface of the top cover 202 above a mirror array of the micromirror device 208.
FIG. 2B (Prior Art) is an expanded cross-sectional view of a corner portion of the top cover 202, the micromirror device 208, the transparent window 206, and the package 204. Wires 210 connect the micromirror device 208 to bond pads. The micromirror device 208 is placed within a cavity formed by the package 204 and the cover 202. The window 206 is placed within the cover 202 and above the micromirror device 208. A corner portion of the packaged device 200 is shown with a small gap or kerf 212.
Over time, as DMD devices become smaller, the dimensions of the area surrounding the packaged devices are reduced. Spacing between adjacent packaged device is smaller and more difficult for nozzles to access. The lumens used to project the optical image is increased to improve the visibility of the image but higher lumens also increases the visibility of any optical artifacts.
Light leaks into the packaged micromirror device 200 through the gap 212. The leaked light may be projected onto a display depending on the angle of incident light and the size of the gap 212 and the quality of the image reduced by the presence of leaked light.
FIG. 3 (Prior Art) shows an image 300 projected from the packaged micromirror device 200. A rectangle 304 is formed by the image 300 of white light emitted from the packaged micromirror device 200. Below the white rectangle 304, a horizontal line 306 is visible. The line 306 is the result of light leaking from the packaged micromirror device 200 through the gap 212.
FIG. 4 (Prior Art) is a cross-sectional drawing of a portion of the packaged micromirror device 200. The height 402 of the cover 202 is approximately 0.65 microns. The height 404 of the package 204 is approximately 0.75 microns. The height 406 of the packaged micromirror device 200 is approximately 1.703 microns. The height 408 of the kerf 212 is approximately 0.303 microns.
It is desired to dispense an epoxy based liquid or similar material in proximity to the cover 202 and as a sealant between the cover 202 and the package 204. Dimensions 402, 404, 406 and 408 are small and dispensing of materials can be difficult.
FIGS. 5A-C (Prior Art) describe an unsingulated wafer 500 with packaged micromirror devices 200.
FIG. 5A is a drawing of a wafer substrate 502 comprising an array of micromirror devices 208. Covers 202 are placed and sealed around each micromirror device 208 to form packaged devices 504. After packaging, scribe lines 506 are completely or partially sawn through the substrate 502. The scribe lines 506 form a boundary for each packaged device 504. If partially sawn, controlled mechanical pressure is applied along the scribe lines 506 to fracture the substrate 502.
FIG. 5B is an expanded top view of a portion of the unsingulated wafer 500. Micromirror devices 208 are enclosed within covers 202. Packaged devices 504 are separated from each other by a width 508 on the horizontal axis and a width 510 on the vertical axis.
FIG. 5C is a three dimensional drawing of a micromirror device 208 which is enclosed within a cover 202. The packaged device 504 with cover 202 has a height 512.
As devices shrink, dimensions 508, 510, and 512 are also reduced. The reduction in space between adjacent packaged devices 504 increases the probability of a notch or gap because typical nozzles are unable to fully access the required locations for fluid coverage. More importantly, a hermetic seal is not formed when the fluid cannot be dispensed correctly.
Encapsulation liquids are viscous and self-leveling. They are typically dispensed using a nozzle placed directly above the part being sealed. The nozzle is attached to an automated system which is programmed to follow a defined path above the substrate. At predefined intervals along the path, material is dispensed through the nozzle. The height of the package and spacing between adjacent packages can cause difficulty in dispensing. Both narrow spacing between adjacent packaged device 504 and the height of the packaged device 504 may prevent a typical nozzle from dispensing material at the required location to form a hermetic seal.
FIGS. 6A-D (Prior Art) show a variety of nozzles used for dispensing material in semiconductor packaging. These nozzles use positive displacement and standard Leur-Lock designs. These nozzles access the plane of the substrate 502 from a vertical direction above the substrate 502 and spray liquid perpendicularly to the plane of the substrate 502.
FIG. 6A is a drawing of a typical nozzle 600 used in semiconductor packaging. The nozzle 600 has a length 602 with an orifice 604 placed where material is dispensed. The nozzle 600 is generally with a width 606. The dispensing orifice has a width 608.
The nozzle 600 is limited by its length 602 which is smaller the than height 512 and unable to access the plane of the substrate 502. The nozzle 600 is also limited by the width of its diameter 606 at the dispensing orifice 604 which is larger than widths 508 and 510.
FIG. 6B illustrates a nozzle 610. Nozzle 610 has a length 612 which is longer than length 602. Nozzle 610 is generally cone-shaped. Nozzle 610 has a larger diameter 614 at the inlet where material is supplied and a smaller diameter 616 at the dispensing end with an orifice 618.
FIG. 6C is a drawing of a nozzle 620. Nozzle 620 has a length 622, which is typically longer than the length 602 of nozzle 600 and may have a length longer than the height 512 of a packaged device 504. Nozzle 620 is generally cylindrical with a diameter 624. The dispensing end comprises an orifice 626 of diameter 628. A longer length 622 and a smaller diameter 624 allow nozzle 620 to more easily access within dimensions 508, 510, and 512.
FIG. 6D illustrates a nozzle 630. Nozzle 630 comprises two cylindrical shapes of diameter 632 attached at a 90 degree angle between the supply inlet 634 and the dispensing end 646. The portion 638 between the supply inlet 634 and the 90 degree bend in the vertical direction has a length 640. The portion 644 between the 90 degree bend and the dispensing end 646 in the horizontal direction has a length 648. The dispensing end 646 comprises an orifice 648.
In nozzle 630, the liquid is dispensed horizontally and parallel to the plane of the wafer substrate. Horizontal dispensing is an advantage in directing the fluid to the cover edge. However, the length 648 is longer than dimensions 508 and 510, and nozzle 630 is unable to reach within the spacing between adjacent packaged devices 504.